Information storage mediums such as optical disks have been improved to have high density so as to attain larger storage capacity. In order to realize high density in a disk-shaped information storage medium such as an optical disk, it is necessary to reduce a spot diameter of a light beam condensed onto the optical disk. However, a light beam is radiated from a semiconductor laser element, which is typically used as a light source, in such a manner that a radiation angle in a direction vertical to a composition surface of a laser chip (vertical direction) and a radiation angle in a direction horizontal to the composition surface of the laser chip (horizontal direction) are different. The radiation angle in the vertical direction is about twice to three times larger than the radiation angle in the horizontal direction. Therefore, light volume of the light beam cannot be efficiently used if the light beam is condensed directly, because the light beam makes an elliptical shape spot when the light beam is condensed directly. Moreover, a focal point in the horizontal direction and a focal point in the vertical direction do not match if the light beam is condensed directly, because positions of a light emitting point are actually different (displacement of an actual light emitting point).
To solve the problems above, as shown in FIG. 13, a method for shaping the spot of the light beam into a substantially round shape is suggested, in which a shaping prism 18 is provided on an optical path of a light beam 11 radiated from a semiconductor laser element (not shown) in order to expand the light beam in one direction or narrow the light beam in one direction by the shaping prism 18. According to the arrangement above, however, there is a problem that an optical pickup as a whole becomes large because a size of the shaping prism 18 cannot be sufficiently reduced, and because at least two shaping prisms 18 must be used in order to keep the optical path of the light beam 11 parallel before and after the light beam 11 is shaped.
Thus, WO98/50913 (Publication Date: Nov. 12, 1998), as shown in FIGS. 14 and 15, suggests a method for matching a beam diameter in the horizontal direction with a beam diameter in the vertical direction. In the method, the beam diameter in the horizontal direction is matched with the beam diameter in the vertical direction by using a shaping element 19 including two holograms 20 in order to change the radiation angles of the light beam 11. According to the arrangement above, it is possible to shape the light beam 11 having an elliptical shape cross-section into a substantially round shape by adjusting a position of the shaping element 19 in a plane vertical to an optical axis and along the optical axis direction. Moreover, it is possible to reduce a size of the optical pickup as a whole because the shaping element 19 is an optical element having a plain plate shape.
However, according to the art disclosed in above WO98/50913, patterns of the holograms 20 of the shaping element 19 are in oval shape. Thus, the position of the shaping element 19 must be adjusted in a horizontal direction and in a perpendicular direction in the plane vertical to the optical axis in order to match the shaping element 19 with the optical axis of the semiconductor laser.
Moreover, it is necessary to adjust distance between the shaping element 19 and the semiconductor laser in accordance with property of each semiconductor laser because each semiconductor laser produced has different wavelength and amount of displacement of the actual light emitting point, thereby causing inter-individual errors (inter-individual unevenness). In other words, it is necessary to perform positional adjustment along the optical axis direction, in addition to the positional adjustment in the horizontal direction and in the perpendicular direction in the plane vertical to the optical axis. Thus, the positional adjustment must be performed in three directions in total.
Hence, the shaping element 19 of WO98/50913 must include a jig for moving the shaping element 19 in the optical axis direction in addition to a jig for moving the shaping element 19 in directions in the plane vertical to the optical axis. Similarly, when the shaping element 19 and the semiconductor laser element are unitized, it is necessary to adjust the distance between the shaping element 19 and the semiconductor laser element on a package (that is the positions of the shaping element 19 and the semiconductor laser element in the optical axis direction) in accordance with the amount of displacement of the apparent light emitting point of each semiconductor laser.
In short, for example, when a combination of the shaping element 19 and the semiconductor laser element (e.g. the optical pickup) is used, there is a problem that time and costs for the positional adjustment in the optical axis direction are needed irrespective of whether or not the shaping element 19 and the semiconductor laser element are unitized.